A light-emitting diode (LED) is a semiconductor light source. An LED includes semiconducting materials doped with impurities to create a p-n junction, in which electrical current can easily flow one directionally from the p-side (anode) to the n-side (cathode), but not in the reverse direction. Charge-carriers (e.g., electrons and holes) flow into the p-n junction from connecting electrodes at each end of the junction having different voltages. For example, when an electron combines with a hole, the electron falls into a lower energy level and can release energy in the form of a photon, e.g., emitting light. This effect is referred to as electroluminescence. The wavelength of the light emitted, and thus the color of the emitted light, depends on the band gap energy of the materials forming the p-n junction. For example, bright blue LEDs are based on the wide band gap semiconductors including GaN (gallium nitride) and InGaN (indium gallium nitride). For producing white light using LEDs, one technique is to use individual LEDs that emit three primary colors (red, green, and blue) and then mix all the colors to form white light. Another technique is to use a phosphor material to convert monochromatic light from a blue or ultraviolet LED to broad-spectrum white light, e.g., in a similar manner to fluorescent light bulbs.
Semiconductor LEDs and semiconductor laser diodes (LDs) are known for their energy efficiency in converting electricity into light and can be used to construct energy-efficient lighting sources or devices for a wide range of applications. For example, LEDs have been adopted in various lighting devices to replace or substitute fluorescent and candescent lights. LED lighting devices can be designed based on DC power supplies such as batteries or AC power supply.